Not very, if you know what you're doing.
See if they'll buy you another one to use as a lab device. I'd strongly recommend against learning on your production device, particularly if you're unfamiliar with Cisco's nomenclature.
If they will not, you could try
GNS3 to emulate at least the commandline of a PIX. You wouldn't get to use the web GUI, and I like ASDM, but at least you'd get some experience with it.